1. Field of the Invention
This invention relates to compositions containing high-molecular weight siloxanes with multiple (meth)acrylate functional groups and reinforcing fillers, which can be crosslinked by radiation to give elastomeric coatings useful for a variety of applications.
2. Description of the Related Art
Silicone rubbers are versatile materials useful in a variety of applications. The most common method of producing silicone rubbers is by compounding gums of high molecular weight polyorganosiloxanes, filler, processing aids and peroxide curing agents and then curing at elevated temperature (150° C. to about 250° C. depending on the peroxide).
In order to obtain uniform rubbers, the gum, fillers and additives have to be mixed thoroughly e.g. in a Banbury or Bread Dough mixer. The peroxide curative can then be added on a two-roll mill before the rubber is press molded, Hot Air Vulcanized, oven-cured, or steam-cured. See W. Lynch, “Handbook of Silicone Rubber Fabrication,” Van Nostrand Reinhold Co., NY, 3 97. However curing and processing of these gums is difficult and slow.
Another form of silicone rubber is known as room temperature vulcanizing silicone rubbers. This type of product, usually designated as RTV's, employ liquid low molecular weight polyorganosiloxanes, crosslinkers, fillers and catalysts. They are available as one part and two part systems. The one part system normally consists of a hydroxyl terminated polyorganosiloxane, triacetoxy or trimethoxy containing silanes as crosslinkers, organometallic catalysts and fillers. The curing is effected by moisture. The two part system comprises a silicon hydride crosslinker in one part, and a vinyl containing silicone, platinum catalysts, and fillers in the second part. Upon mixing the two parts, the curing takes place.
Both types of RTV's generally display similar properties to the heat cured silicone gum rubbers. Their viscosities range from easily pourable liquids to thixotropic pastes, therefore, they are useful as adhesives, coatings and sealants.
However, a complete curing to form rubber usually takes several hours or even days at room temperature, although cure time can be reduced by using elevated temperatures.
Silicone compounds calendared onto glass cloth, fabric, or other substrate may be cured with one of several common thermal peroxide catalyzed processes. Two of the most popular curing processes are Autoclaving and Rotocure.
Usually expensive polyester interlayers are required to prevent backside/frontside bonding which, if not used, would be a problem for calendared rolls cured in an autoclave. In addition, autoclave curing of rolls takes 12 to 16 hours at the appropriate peroxide temperature; and, the cure achieved is not always uniform across the width of the roll.
In the Rotocure process, the peroxide catalyzed silicone calendared glass cloth, fabric, or other substrate is brought and kept into contact with a large diameter heated metal cylinder. Typical peroxide cures take three to five minutes to achieve. A Rotocure drum with a diameter of five feet would require a drum speed of less than six to ten feet per minute to achieve a satisfactory cure.
U.S. Pat. No. 4,587,159 describes silicone coated fabric made by coating with a silanol-terminated polydimethylsiloxane and an aluminum catalyst, followed by thermal treatment. However, these condensation curing systems suffer from slow curing times, lower line speeds and the need for a subsequent oven post-curing.
U.S. Pat. Nos. 5,004,643, 5,209,965 and 5,418,051 describes a process for treating porous webs, e.g. fabric, to produce internally silicone-coated webs, using RTV-type silicone compositions containing hydrogen siloxanes, vinyl siloxanes and a platinum catalyst. U.S. Pat. No. 4,666,765 describes silicone-coated fabrics containing different coats made from compositions containing hydrogen siloxanes, vinyl siloxanes and a platinum catalyst. However, these platinum-cured systems also suffer from slower curing times and do often require extended oven post-cure times, thus not allowing high production rates.
Thermal processes usually require peroxide or other catalysts, often resulting in the need for subsequent post-curing to remove the volatile, and sometimes corrosive, by-products of standard heat-cured processes.
Since usually supplemental curative materials are necessary in these processes, compounding can be cumbersome, pot-life and compound stability are limited, thus resulting in increased amounts of scrap material.
In the area of Conformal Coatings for use in electronic and optical devices, platinum-catalyzed systems typically use expensive metering and mixing equipment which requires shut-down and clean-out when making product changeovers. Formulation is also sensitive and usually requires that such formulation be done at the silicone supplier rather than at the user due to potential loss of catalyst activity, cross-contamination problems and other mixing errors.
It is desirable therefore that silicone compositions, which may be easily processed and handled and are capable of very rapid curing at or near room temperature to elastomeric silicone rubbers useful as coatings of fibres, webs and sheet-like carriers, be developed.
Radiation-curable silicones bearing (meth)acrylate groups have been known to show excellent curing speed and properties. They are widely used as release-coatings, for example for paper substrates.
U.S. Pat. No. 4,978,726 describes siloxanes with terminal acrylate groups which are useful as radiation-curable coating materials for sheet-like carriers.
U.S. Pat. No. 5,494,945 describes siloxanes with terminal and/or pendant acryloxyalkyl groups with chain lengths of less than N=330 useful for release coating applications.
U.S. Pat. No. 5,391,405 describes branched siloxanes with terminal and/or pendant acryloxyalkyl groups for release coating applications.
U.S. Pat. No. 5,494,979 descibes compositions for abhesive coatings containing additives made from acrylated MQ resins and siloxanes having terminal and pendant acrylate groups but <100 unsubstituted dimethylsiloxane units.
U.S. Pat. No. 5,863,966 describes siloxanes with multifunctional (meth)acrylate headgroups for use in radiation-curing printing inks.
However in order to make coatings with elastomeric properties, the presence of longer uninterrupted polydimethylsiloxane segments and fewer acrylate groups is desired, which has adverse effects on the curing speed.
U.S. Pat. No. 4,675,346 describes UV-curable silicone rubber compositions containing siloxanes with two terminal (meth)acryloxypropyl groups, and fillers. It teaches that such siloxanes become uncurable for long-chain silicones (>50000 m.w).
U.S. Pat. No. 4,929,647 describes radiation curable compositions containing acrylate-functional siloxanes groups and SiH-containing MQ resins. The siloxane contains only two acrylate groups which are present in the terminal positions of the polymer chain.
U.S. Pat. No. 4,963,438 describes siloxanes with terminal and/or pendant acryloxyalkyl groups for applications as radiation-curable adhesive coatings. However, linear materials of that type have no more than 2000 dimethylpolysiloxane units.
U.S. Pat. No. 4,940,766 describes siloxanes with terminal and/or pendant acryloxyalkyl groups with chain lengths up to N=˜500.
EP 0237757 describes UV curable elastomer coatings from αω (meth)acryloxy-alkylfunctional siloxanes which also have mercapto groups. These elastomers however have viscosities of less than 50000 mPas and do not have the desired long chain lengths displayed by the siloxanes used in this invention. In addition, mercaptofuntional siloxanes are more expensive, difficult to prepare and handle, and have an undesirable odor.
U.S. Pat. Nos. 4,575,546 and 4,575,545 describe radiation-curable silicones having multiple (meth)acryloxyalkyl-functional siloxane units clustered near the end of the chain ends. They are made in a complicated and expensive process via the polymerisation of methyl and vinyl functional trisiloxanes, followed by hydrosilylation with acrylate-functional SiH functional siloxanes.
U.S. Pat. No. 5,091,483 describes radiation-curable elastomers made from siloxanes with a chain length of less than ˜1000 units which contain terminal monoacrylate groups bound via an amide-containing spacer group. These compounds suffer from a lengthy and uneconomical synthesis, and lack the multiple acrylate groups required for fast curing of high molecular weight systems. U.S. Pat. No. 6,211,322 describes organosilicon compounds comprising terminally and/or laterally at least one Si—C-bonded organic radical which has at least two (meth)acrylate groups and optionally also monocarboxylic acid groups, free from double bonds, attached by way of primary hydroxyl groups, and their preparation. However, no linear materials of that type with more than 1000 dimethylpolysiloxane units are described.